Several control systems have been developed to control the diameter of a monocrystalline rod being pulled by the CZ method. These involve optical scanning systems and CCD camera systems that derive crystal diameter information from a reflective meniscus around the crystal's emergence from the melt surface. Although these systems work fairly well in controlling the diameter of the body of the crystalline rod, improved precision is always welcome, especially since the completed rod must be ground to a uniform diameter before it is sliced into wafers.
The existing systems have significantly more difficulty in accurately controlling the diameter of the neck of the crystal, which is formed as the pulling process is initiated. The crystal growth from a seed crystal is necked down to a diameter of 3 to 4 millimeters before the diameter is enlarged to a shoulder and thereafter kept constant as the cylindrical body of the rod is pulled. Forming larger diameter cylindrical bodies of 300 millimeters as desired by the wafer processing industry requires that the neck of the crystal be made somewhat larger, at a diameter of about 5 millimeters. One of the functions of the crystal neck forming process is to reduce crystal defects in the early stages of crystal growth. Larger neck diameters are less effective at reducing defects, but defect reduction is improved by maintaining well-controlled diameters in the crystal neck region. Thus, improving the accuracy of diameter control of the neck region of the crystal becomes especially important for reducing defects in necks that are necessarily somewhat larger in diameter, because of the increased weight the necks must support as the diameter of the body of the crystal being pulled increases.
The improvements we have devised in a crystal diameter control system especially improve the accuracy of the diameter maintained in the neck region of the crystal. Our system also improves on the accuracy and versatility of the diameter control of the cylindrical body of the crystalline rod.